The present investigation analyzed the ability of tetrahydrobiopterin (BH4) and 6-methyltetrahydropterin (6 mPH4) to penetrate into rat brain tissue following peripheral administration. The drugs were given i.p. at a dose of 100 mg/kg. Following injection of BH4 or 6 mPH4, 5 different areas of rat brain were removed at 5 different time periods to be analyzed for levels of BH4 and 6 mPH4: striatum, hippocampus, hypothalamus, cortex and cerebellum: time points - 0, 30, 60, 90, 120 and 150 min. post injection. At present we see no change in the endogenous levels of striatal BH4 following i.p. BH4. However, there is a significant increase in 6 mPH4 following its i.p. injection. We subsequently examined the effect of 6 mPH4 injection on endogenous dopamine synthesis by monitoring levels of the metabolites DOPAC and dopamine. There was only a small apparent effect of 6 mPH4 injection on DA and DOPAC.